Normal body cells grow, divide, and die in an orderly fashion. During the early years of a person's life, normal cells divide more rapidly than needed so that the person will grow until adulthood. After adulthood, cell division in most parts of the body is in balance, with cells dividing to provide only enough new cells to replace worn-out or dying cells and to repair injuries.
Cancer arises from a loss of normal cell division control. In cancerous tissue the cell division balance is disrupted and more new cells than needed to replace worn-out or dying cells are provided. This disruption can result from uncontrolled cell growth or loss of a cell's ability to undergo apoptosis or cell destruction.
The gradual increase in the number of dividing cancer cells creates a growing mass of tissue called a tumor or neoplasm. As more and more of these dividing cells accumulate, the normal organization of the tissue gradually becomes disrupted. Also, during tumor growth the biological behavior of cells slowly changes from the properties of normal cells to cancer-like properties. Some characteristic traits of cancer tumors are an increased number of dividing cells, variation in nuclear size and shape, variation in cell size and shape, loss of specialized cell features, and loss of normal tissue organization.